1. Field of the Invention
This Invention relates to a liquid crystal display that is frequently used for conventional electronic displays such as monitors of televisions and computers. More particularly, the invention relates to a liquid crystal display suitable for a sequential color illumination method, which is called "field sequential color display" as a technical term.
2. Description of the Related Art
Although conventional liquid crystal displays (abbreviated as LCD hereinafter) had been usually used for monochromatic displays, expressing arbitrary colors was made possible since a method for controlling transmittance of image elements for each color with a liquid crystal by attaching micro-color filters to each image element in a liquid crystal cell (micro-color filter method) was developed (Handbook of Liquid Crystal Device (1990), p492. edited by 142th Commission Panel of the Science Council of Japan, published by Nikkan Kogyo Shinbun-sha).
The color filter method involves, however, problems as follows:
(1) Two third or more of incident beam is wasted by being absorbed by color filters for each color; for example 70 to 90% of light energy is wasted since a red color filter absorbs blue and green spectra. PA0 (2) Triple loss in resolution and driving circuits is inevitable since one image element is composed of three color pixel of red, green and blue.
For solving these problems, a method called a sequential color illumination method (a field sequential color method) has been developed in which three color images are sequentially displayed without attaching color filters to each image element (Liquid Crystal Display Technology (1996); by Shoichi Matsumoto. p50, published by Sangyo Tosho Co.).
To avoid twinkling to the eye due to color switching (flickering) in the sequential color illumination method described above, three colors should be switched within about 1/60s that is one flame time (image display time for a set of three colors), i.e., about 1/180s or about 6 ms, per one color. The response time of the liquid crystal should be about 3 ms or less provided that, for example, one-half of the time above is allotted for switching of each image element, or response of liquid crystals, and a back-light is turned on during remaining one-half of the time.
However, a LCD by a sequential color illumination method having a high response speed, being able to display half-tone images and being easily produced has not yet been realized.
For example, a TN type (twist nematic type) liquid crystal cell (abbreviated as TN cell) currently used for a high quality active matrix (AM) LCD can not be used for the sequential color illumination method because, though it depends on the required half-tone state, its response time is as long as 20 to 100 ms. When the liquid crystal cell is forcibly applied for the low speed sequential color illumination method, the quality of display is largely deteriorated due to the occurrence of flickering. The response is far more delayed in STN type (super twist nematic type) liquid crystal cells practically used in the simple matrix method, being 50 to 300 ms in the response times While a ferroelectric liquid crystal cell has a high response speed, it has some problems that half-tone images can be hardly displayed, cell gaps should be made very thin and a troublesome orientation treatment is required.
The inventors of this invention proposed a liquid crystal display element using a bend orientation liquid crystal cell (including a liquid crystal cell in which a twisted orientation exists at the center of the cell; abbreviated as a bend cell hereinafter) and a phase compensation plate (a phase difference plate) in Japanese Unexamined Patent Publication No. 7-84254. In this element, a biaxial phase compensation plate that three-dimensionally compensates retardation of liquid crystal cells is placed on a bend cell to suppress visual angle dependency, thereby widening visual angles of the cell. Since the bend cell can be driven by a similar voltage as that of the TN cell by designing the phase compensation plate to optically compensate orientation of liquid crystal under a low voltage, the element can be practically manufactured by a conventional production process. As shown in FIG. 8, tile response time of this bend cell (cell gap of 8 .mu.m) is 2 to 8 ms, which is as small as 1/10 or less than that of the TN cell described above (the response time is 10 times faster). However, since the response time becomes far more larger than 3 ms depending on the switched half-tone levels (gray level), this type of cells are not yet sufficient for composing displays for the sequential color illumination method.